The present invention relates generally to data transmission in mobile communication systems and, more specifically, to channel estimation.
As used herein, the terms “user equipment” and “UE” can refer to wireless devices such as mobile telephones, personal digital assistants (PDAs), handheld or laptop computers, and similar devices or other User Agents (“UAs”) that have telecommunications capabilities. In some embodiments, a UE may refer to a mobile, wireless device. UE may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station or access device transmits signals throughout a geographical region known as a cell. As the technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems/equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” or “access device” will refer to any component, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UE with access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, a base station provides radio access to one or more UEs. The base station comprises a scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all UEs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UEs, deciding the transport channel for each UE's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UEs through a control channel.
In LTE systems, within a communication channel, the base station transmits data to UEs using resource blocks or other encapsulated-data formats. Each resource block includes multiple resource elements (REs) arranged in a number of columns and a number of rows as known in the art (see, for example, FIGS. 1a and 1b). The position of each RE within the resource block corresponds to a particular time/frequency combination within the resource block. The frequency width of each RE can be referred to as a subcarrier or a frequency interval while the time duration of each RE can be referred to as a time interval. A piece of data being transmitted in each RE can be referred to as a data portion. The combination of data portions in each time interval can be referred to as an Orthogonal Frequency Division Multiplexed (OFDM) symbol. As such, a time interval can be referred to as an OFDM symbol interval.
During communication, the resource blocks may carry various reference signals to facilitate communication between a base station and a UE. The reference signals can be used for several purposes including distinguishing the several different communication modes that may be used to communicate with UEs, channel estimation, coherent demodulation, channel quality measurement, signal strength measurement, etc. Reference signals are generated using data known to both a base station and a UE, and can also be referred to as pilot, preamble, training signals, training symbols, training sequences, or sounding signals.
A UE uses the reference signals to estimate communication channel conditions between the UE and the base station. Because reference signals transmitted by a base station are initially transmitted using predetermined values, after receiving a reference signal transmission, the UE can compare those predetermined values to the reference signal values actually received from the base station. By analyzing how the reference signal values were modified or distorted by the communication channel, the UE can estimate the communication channel conditions between the UE and base station and, using that estimate, implement channel compensation to ensure transmissions from the base station are accurately received by the UE.
Exemplary reference signals for facilitating channel estimation include a cell-specific or common reference signal (CRS) that is sent by a base station to UEs within a cell and is used for channel estimation and channel quality measurement, a UE-specific or dedicated reference signal (DRS) sent by a base station to a specific UE within a cell that is used for demodulation of a downlink channel, a sounding reference signal (SRS) sent by a UE that is used by a base station for channel estimation and channel quality measurement, and a demodulation reference signal sent by a UE that is used by a base station for demodulation of an uplink transmission from the UE.
After performing channel estimation, the UE can demodulate data received from the base station. To ensure efficient data transmission, LTE downlink communications may adopt high-order spectrum efficient Quadrature Amplitude Modulation (QAM) schemes, i.e., QPSK, 16QAM, and 64QAM. The demodulation performance of high-order QAM schemes can be affected by the quality of the communication channel estimation. As such, a good channel estimation scheme for LTE downlink demodulation is important for optimal network performance.
LTE networks may specify a particular set of cell specific reference symbols, known as pilot symbols or pilots. The pilot symbols facilitate channel estimation and are scattered two-dimensionally (2D) throughout both the time-direction and the frequency-direction of a resource block. Because the pilot symbols are distributed in two-dimensions, pilot-symbol-aided channel estimation schemes and 2D channel estimation schemes are used in LTE network implementations.
FIGS. 1a and 1b are illustrations of subframes showing example time-frequency locations of cell specific reference symbols (i.e., pilots) for a first antenna within resource blocks in an LTE system. In other implementations, the pilot locations can be shifted by any number of subcarriers (e.g., 3) depending upon the system configuration. FIG. 1a shows pilot symbols distributed in subframes or resource blocks using a normal cyclic prefix, while FIG. 1b shows the pilot symbols distributed in subframes using an extended cyclic prefix. In each of FIGS. 1a and 1b, three subframes 100, 102 and 104 are illustrated. Each subframe is comprised of multiple REs represented by each individual square within each subframe. Each subframe includes 12 subcarriers or frequency intervals (the horizontal collection of REs running through each subframe in the time direction) transmitted over a total frequency band, with each subcarrier being transmitted at a single frequency over time. Each subcarrier corresponds to a single frequency interval.
In each subframe shown in FIGS. 1a and 1b, in the frequency direction (e.g., moving vertically within each subframe), the number of subcarriers excluding the direct current (DC) subcarrier depends on the channel bandwidth configuration but may generally be an integer multiple of 12. In the time direction, there are 14 OFDM symbol intervals or time intervals per subframe for the normal cyclic prefix configuration (see FIG. 1a) and 12 OFDM symbol intervals or time intervals per subframe for the extended cyclic prefix configuration (see FIG. 1b). Each OFDM symbol interval includes a vertical slice of REs of each subframe and includes REs at different frequencies (i.e., across different subcarriers). In LTE network implementations, each subframe is 1 ms wide and each RE is 15 kHz (or 7.5 kHz) wide in frequency and one OFDM symbol interval long in time.
Each subframe or resource block includes a number of pilot locations 106. In the examples shown in FIGS. 1a and 1b, the pilot location pattern includes 4 pilot locations 106 distributed within each of subframes 100, 102 and 104. The pilot location patterns repeat across both the time and frequency directions within a subframe and repeat within each subframe. When receiving downlink transmission from a base station, the UE uses the pilot symbols at pilot locations to perform channel estimation and, ultimately, demodulation of the downlink data transmissions.
When performing channel estimation, for an LTE UE, the received signal in the RE having subcarrier index k and OFDM symbol index s can be written as:r(k,s)=H(k,s)x(k,s)+z(k,s)  (1)
In equation (1), H(k, s) is the frequency-domain channel state at the resource element (RE) location (k, s) in the subframe of interest, x(k, s) is the transmitted symbol, z(k, s) is the additive white Gaussian noise (AWGN) with zero mean and variance σ2 and r(k, s) is the received signal. Generally, a UE performs data demodulation on a subframe by subframe or resource block by resource block basis over a number of subcarriers (e.g., in LTE the number may be an integer multiple of 12) that are allocated to the UE.
During data demodulation, the UE uses the transmitted downlink pilot symbols to perform downlink channel estimation to allow for coherent demodulation of signals transmitted from the base station. The original values of the transmitted pilots (or reference signals) x(k′, s′) with normalized energy are assumed to be known beforehand by both the base station and the UE, where k′ and s′ are respectively the subcarrier index and the OFDM symbol index for the received pilot symbols. The UE can then compare the received and known pilot symbols to generate the channel estimation at the pilot locations. The raw channel estimates at the pilot locations are given by equation (2).{tilde over (H)}(k′,s′)=r(k′,s′)/x(k′,s′)=H(k′,s′)+n(k′,s′)  (2)
In equation (2), {tilde over (H)}(k′,s′) denotes the raw channel estimate at subcarrier k′ and OFDM symbol s′, and n(k′, s′) is the background noise with zero mean and variance σ2.
In the present disclosure, for notational simplicity, Sk′={k′1, k′2, . . . , k′Nk} denotes the pilot symbol subcarrier index set with NK indices (the set is the same at any given OFDM symbol with pilots) and Ss′(t)={s′t−Ns+1, s′t−Ns+2, . . . , s′t} denotes, at time instant t, the pilot symbol's OFDM symbol index set with Ns indices for the recently received Ns pilot OFDM symbols . The OFDM symbol index is enumerated from s′0 at t=0 and it is presumed that the raw channel estimates at the pilot locations up to the OFDM symbol s′t are available. As such, the raw channel estimates of the pilot symbols in one or several previous subframes can be used. Also, it is presumed that no raw channel estimates at the non-pilot locations of the previous subframes are available.
Channel estimation techniques need to take into account the channel characteristics for a good performance. In a slow time-varying channel, the channel state realization over a particular time period may not change significantly. A channel estimation technique configured to use the extended time period (with minimal variation) and long-term averages when performing channel state estimation may result in inaccuracies due to short-term changes in the channel state condition causing inaccurate channel estimates. On the other hand, channel estimation techniques should not be too complex. A channel estimation technique with overly complex processing may cause substantial delay in the UE and excessive processor and battery power use.